The present invention relates to the bonding of carbon structures to metal. In particular, the invention is related to methods of bonding of carbon structures to metal and composite structures formed according to this method which are particularly useful as capacitor electrodes, and may also be useful in a variety of other electrical, electronics, or structural applications.
Carbon is commonly used in a variety of forms as an electronic conductor in battery and capacitor electrodes. In order to extract current from the battery or capacitor it is necessary to attach a current collector, typically composed of a metal such as aluminum or copper, to the electrode. One method presently used to bond carbon to aluminum requires the carbon source, for example carbon black, carbon fibers, carbon aerogel, etc., to be ground into a fine powder. The powder is then mixed with an organic binder and a solvent carrier. This mixture in the form of a slurry is then coated on to the aluminum substrate that will serve as the current collector. A similar method is used to bond an electrode composed of compressed carbon to an aluminum current collector, except that the carbon is already in a powder form.
This method suffers from two main problems: coating adhesion and contact resistance. The organic binder serves two purposes, first it binds the coatings to the aluminum substrates, and second it binds the carbon particles (e.g., from aerogel or compressed carbon) together. The coating adhesion suffers when attempts are made to maximize the carbon contents in the electrode. For a given coating thickness, more carbon means that less binder may be used, and therefor less adhesion is obtained to the aluminum current collector substrate. The problem of contact resistance arises from the fact that the carbon particles can only make electrical contact at points where the particles is in mechanical contact with the aluminum substrate, and even then the particle must penetrate the oxide layer on the aluminum.
Many applications of capacitors require very low equivalent series resistance (ESR), for minimal insertion loss, high turn-around efficiency, and high frequency performance. Electrochemical double layer capacitors (EDLCs) have been prevented from being used in many traditional capacitor applications because a low-cost, mass produceable method for making a low resistance contact suitable for use in EDLCs has not been available.
One existing method for making a low resistance carbon contact is to place a sheet of commercially available carbon paper (e.g: from Lydol) on each side of a larger area sheet of inconel expanded metal (Exmet). This sandwich is then saturated with a precursor mixture of resorcinol-formaldehyde, used for making carbon aerogel. The saturated sandwich is then cured at about 90xc2x0 C. and then pyrolyzed at 1050xc2x0 C. to form an aerogel. This results in the active carbon aerogel being in intimate contact with the Exmet current collector, providing low resistance contact. The electrode assembly may then be spot-welded to the foil current collector.
While this method achieves a desired technical result, it has the drawback that both the materials and process arc expensive. Each electrode requires two sheets of carbon paper and one sheet of Exmet, plus the labor required to assemble the components. Moreover, the extra sheet of carbon paper, and the Exmet, result in excess thickness for most ESR applications.
Another approach which has been attempted to make good electrode contact between a carbon electrode and a metal current collector is flame or plasma spraying. These are well-known techniques for applying refractory metals and other coatings to metals and ceramics (see, for example, Technical Bulletin issued by METCO Inc. of Westbury, N.Y., issued in April 1984, the disclosure of which is incorporated by reference herein for all purposes). However, the use of flame or plasma spraying to apply metals to carbon structures for the purposes of forming low resistance contacts has met with unacceptable results. One possible explanation for these poor results is that the very different coefficient of thermal expansion of the carbon structures and the metal used to coat them result in poor adhesion of the carbon to the substrate and shearing at the carbon-metal interface on cooling.
Accordingly, a technique to produce a low resistance contact between a carbon structure and a metal would be useful in the development of improved capacitor and battery electrodes as well as in other electrical, electronic, and structural applications.
To achieve the foregoing, the present invention provides a technique in which a carbon structure is first coated with a tie layer as a intermediate prior to the deposition of metal on a carbon substrate. The tie layer is composed of a material with structural and chemical affinity for both carbon and metal. In a particularly preferred embodiment, the tie layer is formed by a high temperature deposition of silicon on the carbon structure, preferably by plasma deposition. A metal such as aluminum, nickel or copper, is then flame or plasma sprayed on to the carbide-coated carbon structure and alloys to the silicon and silicon carbide.
Thereafter, the silicon and metal sprayed carbon structure surface may be placed in contact with a metal substrate and heated to alloy the substrate to the previously spray-deposited silicon and metal material on the carbon structure surface. Capacitor electrodes formed according to this technique show very low Equivalent Series Resistance (ESR) and improved capacitance at high frequencies.
In one aspect, the invention provides an electrode having a metal-carbon laminate structure, including a carbon layer, a metal layer, and a tie layer between the carbon and metal layers. The tie layer is composed of a material with structural and chemical affinity for both carbon and the metal.
In another aspect, the invention provides a method of making an electrode. The method involves forming a tie layer on a carbon layer, the tie layer composed of a material with structural and chemical affinity for both the carbon and a metal, and bonding a metal layer to the tie layer-coated carbon layer.
In another aspect, the invention provides a metal-carbon laminate structure. The structure includes a carbon layer, a metal layer, and a tie layer between the carbon and metal layers, the tie layer composed of a material with structural and chemical affinity for both carbon and the metal.
In another aspect, the invention provides a method of bonding metal to carbon. The method involves providing a carbon substrate, forming a silicon carbide tie layer on a surface of the carbon substrate, and bonding a metal layer to the tie layer.
In yet another aspect, the invention provides a carbon fiber reinforced metal structure. The structure includes carbon fibers interspersed with metal, said carbon fibers coated with a silicon composition including silicon carbide.
In still another aspect, the invention provides a method of making carbon fiber reinforced metal structure. The method involves forming a silicon coating on carbon fibers and casting the coated carbon in a metal casting in which the temperature is high enough to form an alloy between the metal and the silicon, thereby forming an carbon/silicon metal matrix.